It is known to incorporate short lengths of steel wire in concrete to improve its properties. Such a reinforced composite is described in the specification of Australian Pat. No. 290,468. A related reinforced composite comprising a castable matrix mass, such as concrete, incorporating discontinuous reinforcing filaments having a non-round cross-section with a width and thickness ratio of not greater than five is disclosed in U.S. Pat. No. 3,650,785.
In each of the above concrete-steel fibre composites, the increase in strength has been limited by the poor mechanical and chemical bond between the steel fibres and the cement matrix. The steel fibres tend to pull out of the matrix long before the fibres reach their ultimate strength.
Attempts have been made to overcome the problem but in each case the modification has been impractical or the bond strength increased only marginally. One modification involved the use of longer steel fibres but such were found too difficult to handle and mix in the matrix. Similarly, the fibres have been chemically treated, coated or crimped in an attempt to increase bond strength but with little success.
German Pat. No. 2,042,881 suggests that the wire ends be bent or shaped, but this tends to reduce mixability and reduces the effective fibre length.
It is the primary object of the invention to provide an improved form of reinforcing element which is formed to improve the locking effect between the element and the matrix it reinforces.
The invention provides, in a first aspect, a reinforcing element for materials such as concrete, mortar, glass, stabilised materials, plastics or ceramics, comprising a discontinuous filament or fibre or reinforcing material having end portions which are larger in cross-section than smallest cross-section of the shank of the filament or fibre.
Preferably, the end portions are larger in both the longitudinal and transverse planar cross-sections thereof.
The interfacial shear stresses in discontinuous filaments or fibres incorporated in a low strength brittle matrix are maximum at the fibre ends. By enlarging the end of the filament or fibre, especially in both cross-sections, the locking effect at the ends is improved due to the enlargement of the ends relative to the shank and accordingly the reinforcing effect of the fibre should be increased using more or all of its available tensile strength.
It is preferred that the end portions of the fibre be enlarged by deformation to suitably shape the end and for this reason the fibre is preferably made from a metal, plastic or other material capable of being permanently deformed or shaped.
Where the material being reinforced is concrete or mortar, the fibres are preferably of steel, although other metals or metallic alloys having the required high tensile strengths, e.g., above 30,000 psi, may be used if desired.
The fibres may have any desired cross-sectional configuration and are preferably in the form of short lengths of wire-like material with the shank thereof plain and of substantially uniform unvarying cross-section. The gauge tensile strength and configuration of the fibre shank is selected according to the desired performance and may include gauges and configurations as disclosed in the prior art referred to hereinabove.
The low strength brittle matrix material may be any of the conventional castable curable or hardenable matrix material but preferably is a cementitious castable matrix mass such as concrete or plain mortar. Most preferably the cementitious matrix mass is based on Portland cement.
Suitable cementitious compositions are disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 4, pages 684-710 (1964), the disclosure of which is hereby incorporated by reference.
The reinforcing fibres, which are discontinuous, may be chosen from a wide variety of materials, including reinforcing glass fibres, reinforcing nylon fibres, reinforcing titanium fibres, reinforcing tungsten fibres, reinforcing copper fibres, reinforcing lead fibres, reinforcing steel fibres and reinforcing aluminum fibres. Of course, alloys of the above-mentioned metals may also be utilized if desired.
The reinforced composite structure of the present invention is made by mixing the reinforcing fibres in the castable matrix. It is preferable to uniformly distribute the fibres throughout the matrix, although some fibre agglomeration can be tolerated. Normally, the reinforcing fibres are used in an amount of from 1/2 to 5% by absolute volume, preferably about 1 - 2% by absolute volume. In the case of Portland cement, concrete or mortar matrixes and steel reinforcing fibres, the steel fibres will be used in an amount of from about 3 - 6% by weight of the total composition.
The castable matrix is then cured or hardened under appropriate conditions, depending upon the nature of the matrix material. A chemical hardener or curing agent may be used in the case of thermosetting polymers. For cementitious products, the normal cure will be a simple time cure (i.e., 1 - 15 days or so) at ambient conditions, or even at elevated temperatures, normally in the presence of moisture.
In a second aspect of the invention, there is provided a method of forming reinforcing fibres of metal or other material capable of being permanently deformed or shaped comprising the steps of passing a narrow strip of said material through a roll forming operation adapted to enlarge edges of the strip and then shearing the strip transversely to form a fibre of the desired width having end enlargements.
Many different roll forming operations may be performed, such as folding or otherwise deforming the edge of the strip or reducing the cross-sectional area of the central portion of the strip to enlarge the edges relative to the central portion. These operations will be described in more detail below.
A preferred method of forming reinforcing fibres comprises forming a plurality of strips simultaneously before the roll forming operation by longitudinally slitting a wider strip. The roll forming and shearing operations are then carried out on the plurality of narrow strips with the strips running in parallel.
A single shearing device may be used to shear said plurality of formed strips simultaneously and preferably comprises a multi-toothed rotary cutter running with a slight clearance against a fixed cutter.
The invention also provides an apparatus for performing the above methods.